This invention relates to liquid crystal display devices. In particular, the invention is concerned with a reflective liquid crystal display device and a transflective liquid crystal display device, employing a single polarizing film method, for effecting bright display in black and white or in color by the agency of a reflector provided in a liquid crystal element of the device, and a sheet of polarizing film deposited on the outer side of the liquid crystal element.
For a reflective liquid crystal display device, there has been mainly adopted a construction wherein a TN (twisted nematic) liquid crystal element or an STN (supertwisted nematic) liquid crystal element is disposed between a pair of polarizing films, and a reflector is installed on the outer side of one of the polarizing films.
With such a reflective liquid crystal display device, however, external light passes through each of two sheets of the polarizing films twice from the time when the external light enters from the visible side of the device until it goes out towards the visible side after reflected by the reflector, so that reduction in light quantity is increased, and thereby a dark display is effected. Moreover, since the reflector is installed on the outer side of a glass substrate of the liquid crystal element, there has arisen a problem that shadows appear on display.
To cope with the problem, a single polarizing film type liquid crystal display device, capable of effecting display with just one sheet of polarizing film, has since been proposed. With such a liquid crystal display device having only one sheet of polarizing film, reduction in light quantity can be decreased in comparison with the case of a conventional reflective liquid crystal display device employing two sheets of polarizing films, thereby improving brightness of images in display.
Further, with the single polarizing film type liquid crystal display device, it is possible to solve the problem of the shadows appearing on display by forming a reflector inside a liquid crystal element.
Such a single polarizing film type liquid crystal display device is comprised of one sheet of polarizing film, one sheet of retardation film, and a liquid crystal element incorporating a reflector, as disclosed in, for example, Japanese Patent Laid-open JP, 04-97121, A.
With such a conventional single polarizing film type liquid crystal display device employing one sheet of retardation film as described above, however, in the case of effecting black display, it has been possible to attain a low reflectance (a ratio of an outgoing light quantity to an incident light quantity as seen from the visible side) for light rays at specific wavelengths only, but impossible to attain a low reflectance for light rays over all wavelengths, so that contrast has been insufficient.
Accordingly, in order to effect excellent black display, development of a single polarizing film type liquid crystal display device employing two sheets of retardation films has been under way, but has not reached a stage as yet where sufficient contrast is achieved.
Also, a single polarizing film type liquid crystal display device employing a compensation layer having a structure twisted in the direction opposite to the twist direction of a liquid crystal layer in place of a retardation film has been disclosed in, for example, Japanese Patent Laid-open Publication JP. 10-123505, A. However, even with introduction of such a construction, it has been difficult to attain a low reflectance for light rays over all wavelengths.
Further, with the single polarizing film type conventional liquid crystal display device described in the foregoing, it is not possible to install a backlight because the reflector does not allow light rays to pass therethrough, so that display can not be seen at places where external light is weak or at night.
Accordingly, there has been developed a transflective liquid crystal display device, employing a half-mirror made of a thin film of aluminum, formed by the vapor deposition method or the sputtering method, or having a reflector provided with an opening for every pixel, so that display is effected by light rays emitted from a backlight at places where external light is weak or at night.
In the case of the single polarizing film type liquid crystal display device, however, a liquid crystal element and optical elements such as a retardation film, and so forth need to be designed such that display in excellent black and white can be obtained by controlling outgoing of reflected light with a sheet of the polarizing film in a state where incident light passes through the liquid crystal element back and forth at the time of reflective display using external light.
On the other hand, at the time of transmissive display using a backlight, since light emitted from the backlight passes through the liquid crystal element only once, the liquid crystal element and the optical elements need to be designed such that display in excellent black and white can be obtained in such a condition as described above by controlling outgoing of reflected light with one sheet of the polarizing film. For these reasons, it has been difficult to obtain high contrast in both reflective display and transmissive display.
A liquid crystal display device having a reflector provided with an opening for every pixel has been disclosed in, for example, Japanese Patent Laid-open JP, 10-282488, A, however, no description on the conditions concerning a liquid crystal element and optical elements has been given therein at all, and no description on how to achieve good contrast at the time of both reflective display and transmissive display has been given therein at all.
The invention has been developed in view of the technical background described above, and an object of the invention is to provide a single polarizing film type liquid crystal display device to realize bright display in high contrast by obtaining excellent black display at low reflectance for light rays over all wavelengths.
Further, it is an another object of the invention to provide a single polarizing film type liquid crystal display device, capable of effecting reflective display by use of external light and transmissive display by lighting up a backlight, and achieving high contrast at the time for both reflective display and transmissive display.
In order to achieve the objects described above, the liquid crystal display device according to the invention is a reflective liquid crystal display device, employing a single polarizing film method, which comprises: a liquid crystal element comprised of a twist aligned nematic liquid crystal layer sandwiched between a first substrate having a reflector and first electrodes, and a second substrate having second electrodes; a twisted retardation film deposited on the outer side (a side thereof, opposite from the side facing the nematic liquid crystal layer) of the second substrate of the liquid crystal element; and further a retardation film and a polarizing film which are sequentially deposited in that order on the outer side of the twisted retardation film.
Further, the invention can also provide a transfiective liquid crystal display device comprising: a liquid crystal element comprised of a twist aligned nematic liquid crystal layer sandwiched between a first substrate thereof, provided with a transflective reflector and first electrodes, and a second substrate thereof, provided with second electrodes; a twisted retardation film deposited on the outer side of the second substrate of the liquid crystal element; further, a first retardation film and a first polarizing film sequentially deposited in that order on the outer side of the twisted retardation film together with a second retardation film, a second polarizing film, and a backlight which are sequentially deposited in that order on the outer side (a side thereof, opposite from the side facing the nematic liquid crystal layer) of the first substrate of the liquid crystal element.
Further, in such case, a third retardation film is preferably installed between the second retardation film and the second polarizing film such that the phase delay axis of the second retardation film crosses the phase delay axis of the third retardation film substantially at right angles, wavelength dependency of a retardation value of the second retardation film differs from wavelength dependency of a retardation value of the third retardation film, and the difference between the retardation value of the second retardation film and the retardation value of the third retardation film is substantially equal to one quarter wavelength.
Or the phase delay axis of the second retardation film may intersect the phase delay axis of the third retardation film substantially at 60 degrees, and a retardation value of the second retardation film may be substantially equal to one quarter wavelength while a retardation value of the third retardation film may be substantially equal to half wavelength.
With the liquid crystal display device having these features, it is preferable that a twist direction of the twisted retardation film is rendered opposite to a twist direction of the liquid crystal element, a twist angle of the twisted retardation film is rendered smaller than a twist angle of the liquid crystal element by a range of 10xc2x0 to 20xc2x0, and a xcex94nd value of the twisted retardation film, indicating birefringent tendency thereof, is rendered smaller than a xcex94nd value of the liquid crystal element, indicating birefringent tendency thereof, by a range of 0.2 to 0.3 xcexcm.
Furthermore, the invention can provide a color liquid crystal display device by providing filters in plurality of colors on either substrate of the first substrate and the second substrate of the liquid crystal element.
Still further, a diffusion film is preferably deposited on the outer side of the second substrate of the liquid crystal element.
The transflective reflector may be a thin metal film with a thickness in a range of 0.03 to 0.01 xcexcm, or a thin metal film provided with an opening defined at every spot corresponding to respective pixels.